In recent years, biotechnology has been remarkably developed. Particularly, in consequence of large-scale genomic analyses accompanying advances in genomic analysis techniques, enormous information of genome sequences has been accumulated. In addition, based on combinations of the above-mentioned information with analyses of various physiological functions, many functional genetic mutations have been found. Analyses of these mutations have been used for genetic diagnoses of human beings as well as improvement of agricultural crops and isolation or creation of useful microorganisms, and thus have greatly contributed to general living.
The mutation analyses are performed by direct analyses of genomic sequences or by use of enzymes that recognize mismatched base pairs. A mutation analysis method comprises detection with a factor capable of binding specifically to a mismatched base pair formed from a mutant-type DNA and a wild-type DNA. A representative example of the mutation analysis method includes detection of mutation sites by use of MutS, MutT, and MutL complexes from Escherichia coli (Patent Literature 1).
A mutation analysis method comprising use of a mismatch endonuclease which specifically cleaves mismatch sites is also known. In the method, a mismatch endonuclease is used to cleave a DNA in the vicinity of a mismatched base pair, and the DNA fragments thus obtained are analyzed to detect the presence or absence and the position of mutations. As a representative example, a method comprising use of a Cell gene product from celery is known (Patent Literature 2), and the method is actually used for analyses of base mutations. However, the enzyme is not heat-resistant, and therefore cannot be used in techniques involving a high-temperature reaction process, such as PCR. Thus, in order to detect base mutations, the method requires four steps of amplification, formation of mismatches, cleavage of mismatches, and detection.
In addition to mutation analyses, examples of biotechnological techniques that have a lot of influence include nucleic acid amplification techniques.
A representative example of the nucleic acid amplification techniques is polymerase chain reaction (PCR) PCR is a technique for easily amplifying a desired fragment of a nucleic acid in vitro. PCR is an experimental technique which is essential in broad fields including the fields of biology, medicine, and agriculture, as well as research regarding genes. PCR is also applied to detection of mutated genes and analysis of methylation of DNA.
Isothermal nucleic acid amplification methods such as a LAMP method and an ICAN method do not require special equipment, and therefore they are used as cheaper methods for detection of nucleic acids.
For structural analyses of the whole genome which have been performed in recent years, a whole-genome amplification method is an important technique, in particular for analyses of scarce samples.
In these nucleic acid amplification methods, incorporation of incorrect bases occurs with a constant probability. The probability has been reduced through improvement of a polymerase or the like. However, the incorporation of incorrect bases still disturbs precise analyses.
The nucleic acid amplification techniques are used not only for amplification of a DNA having a specific nucleotide sequence but also for amplification of a mixture of DNAs having a common nucleotide sequence region at both ends. Specific examples of such nucleic acid amplification techniques include construction of genomic libraries or cDNA libraries. In constructing such libraries, however, a DNA molecule with a higher content is preferentially amplified, which may disturb analyses or screening of various kinds of DNAs.
To solve the above problem, the proportion of a DNA with a higher content is reduced by normalization utilizing self-hybridization (Non-patent Literature 1). SSH-PCR in which PCR and self-hybridization are combined is also used (Non-patent Literature 2). Using these methods, however, DNAs homologous to the DNA with a higher content may be also removed.
In detection of a DNA by a nucleic acid amplification method, a target DNA and a non-target DNA may compete for amplification. In other words, when a non-target DNA is amplified simultaneously with amplification of a target DNA, it is difficult to detect the target DNA. The above problem may be solved by use of real-time PCR in which probes such as cycling probes or TaqMan probes are used to detect only a target DNA. In the case where a non-target DNA exists in an excessively large amount relative to a target DNA, however, it is difficult to detect the target DNA because of false-positive reaction with many similar DNAs.
Such a problem may occur in detection of a small number of mutant alleles in the presence of normal alleles (for example, detection of circulating tumor genes), detection of a small number of methylated or non-methylated alleles by epigenetic assay, detection of a small amount of fetal DNA sequences circulating in the mother's blood, and the like.
To solve the above problem, a method termed restriction endonuclease-mediated selective polymerase chain reaction (REMS PCR) has been developed (Non-patent Literature 3). This method involves use of a heat-resistant restriction enzyme. In this method, a DNA having a mutant nucleotide sequence is selectively detected using primers which, for example, are designed so that cleavage by the restriction enzyme occurs only when a template has a normal nucleotide sequence. Depending on a target DNA to be detected, however, there may be no heat-resistant restriction enzyme having a recognition sequence suitable to detection by REMS PCR.